A Hybrid Energy System Using Cascaded H-bridge Converter
Hui Li1, Zhong Du2, Kaiyu Wang1, Leon M. Tolbert3, Danwei Liu1
1
Dept. of ECE, Florida State University, Tallahassee, FL32310, hli@caps.fsu.edu
Dept. of ECE, North Carolina State University, Raleigh, NC 27695, zdu@ncsu.edu
3
Dept. of ECE, The University of Tennessee, Knoxville, TN 37996-2100, tolbert@utk.edu
2
Abstract∗— Different circuit configurations have been
overvoltage problems resulting from saturation of the
researched to combine clean energy sources and energy storage
transformer in transients; and (5) They are prone to failure
elements. This paper proposes a hybrid energy system to
[6].
integrate the variable-speed wind turbine, fuel cell, and battery
using a cascaded H-bridge converter. One of the advantages of
this topology is that it still can obtain the regulated output
Static
Isolator
voltage if one or more energy sources are diminished. In
addition, the topology can be easily extended to connect more
Transformer
sources without increasing the circuit and control complexity;
therefore, it is beneficial for distributed energy generation.
Different operation modes are analyzed in detail. The control
schemes were developed to extract maximum wind power and
Transformer
charge/discharge the battery with fast dynamics. The
Utility
3-phases
simulation and experimental results are provided to confirm
Fig. 1. Hybrid energy system where power is combined on the
the theoretical analysis.
ac side through a transformer.
Keywords - hybrid energy system, cascaded multilevel converter,
single dc source
In this paper, the outputs from different power sources are
integrated on the ac side by a cascaded H-bridge inverter
I. INTRODUCTION
instead of a transformer. The topology can achieve regulated
As different energy generation technologies become
output voltage at a single source if the rest of energy sources
mature, the power generation systems will employ various
are diminished, resulting in increased robustness. Another
forms of energy generation, storage, and transmission. The
advantage is the topology can be easily extended to connect
power from hybrid energy systems can be combined on the
more sources without increasing the circuit and control
dc side or ac side through different power conversion
complexity; therefore, it is beneficial for distributed energy
configurations [1-5]. A traditional approach to combine
generation and high power applications.
power on the ac side is through a multi-phase transformer,
II. OPERATION PRINCIPLE
shown in Fig. 1. The usage of multi-phase transformer has
the following disadvantages: (1) They are the most
Fig. 2 shows the topology applied to a hybrid energy
expensive equipment in the system; (2) They produce about
system integrating the variable-speed wind turbine, fuel cell,
50% of the total losses of the system; (3) They occupy up to
and battery as an energy storage element. As shown in Fig.
40% of the total system’s real estate; (4) They cause
2, the speed of the generator is controlled through a PWM
difficulties in control due to dc magnetizing and surge
rectifier to extract the maximum wind energy from the wind
turbine. A battery is used as an energy storage element to
∗
This work was supported by funding under U.S. Department of
compensate the stochastic characteristics of wind energy.
Energy, Office of Electricity Delivery and Energy Reliability,
The ac output of the fuel cell energy source and the ac
Award number DE-FG02-05CH11292.
output of wind turbine - generator system can be combined
.
1-4244-0365-0/06/$20.00 (c) 2006 IEEE
Wind Turbine
Vw
Generator
DC Link
PWM
Rectifier
Gear
Box
H-Bridge
Inverter
C
Load
Utility
Grid
Isolated
bidirectional
dc-dc
converter
Battery
Fuel
Cell
Stack
H-Bridge
Inverter
C
Fig. 2. Proposed hybrid energy system using cascaded H-bridge converters.
in series using a cascaded H-bridge inverter to provide
Table I and Fig. 4. From Table I, it can be seen that there are
desired power to the load and to the utility grid even if only
two possible ways to generate −Vdc/2 and Vdc/2. One can be
one source is available and the others are diminished.
used to charge the capacitor while the other can be used to
discharge the capacitor. The criteria required for this
A. Operation modes of cascaded inverter
Fig. 2 can be simplified as shown in Fig. 3 where Energy
Source 1 represents the wind turbine with battery and
Energy Source 2 is the fuel cell stack. Vdc1 can be assumed
to be constant due to the control of the bidirectional dc-dc
converter. Vdc2 can be variable since the fuel cell is
connected to the dc bus directly.
Vdc2
+
S2,1
Utility Grid
S2,2
C2
-
S2,3
S2,4
+
-
Energy
Source 1
Vdc1
+
S 1,1
S1,2
S1,3
S1,4
C1
-
+
Vout
H1
value is chosen large enough so that the variation of its
voltage around its nominal value is small (generally, one can
choose the capacitor-load time constant to be greater than 10
times of the fundamental cycle time), and (3) the capacitor
energy in a cycle. The operation principles of mode II can
be derived similarly.
L
L
A
+
-
voltage is less than the dc source voltage, (2) the capacitance
charging energy is greater than the capacitor discharge
H2
Energy
Source 2
capacitor balancing scheme is that (1) the desired capacitor
L
B
C
-
(a)
v
3/2V dc
V dc
(b)
1/2V dc
θ1
Fig. 3. Simplified hybrid energy system using cascaded H-bridge
topology (a) single phase, (b) three-phase.
θ2 θ3
π
2π
-1/2V dc
-V dc
There are three operation modes for the proposed hybrid
-3/2V dc
energy system: (I) Energy Source 1 standalone mode; (II)
Energy Source 2 standalone mode; and (III) mixed mode of
Fig. 4. 7-level equal step output voltage waveform.
Energy Source 1 and Energy Source 2.
To analyze mode I, a dc source is assumed to be
connected to the first H-bridge (H1) with voltage Vdc, a
capacitor is connected to the second H-bridge (H2) with
capacitor voltage to be held at Vc. For this example, suppose
Vc = Vdc/2 By applying a fundamental switching scheme, the
possible combinations of the two H-bridges that can
generate 7-level equal step output voltage are shown in
Table I. Output voltages for an equal 7-level converter
0 ≤ θ < θ1
θ1 ≤ θ < θ2
θ1 ≤ θ < θ2
θ2 ≤ θ < θ3
θ3 ≤ θ < π/2
v1
0
0
Vdc
Vdc
Vdc
v2
0
Vdc/2
-Vdc/2
0
-Vdc/2
v = v1 + v2
0
Vdc/2
Vdc/2
Vdc
3Vdc/2
There are two situations of mode III. For the purposes of
total electric charge on the capacitor is decreasing, and Q <
discussion, Vdc1 > Vdc2 is assumed. If the situation is
0 means the opposite way. Equation (2) can be rewritten as
reversed, the control situation is similar.
the following formula considering the interval where S2(θ)
(1) Situation 1: Energy Source 1 and Energy Source 2 both
= 0:
have enough available power to maintain the dc bus voltages
Q=
Vdc1 and Vdc2 near constant. The cascaded H-bridge
converter produces 7 voltage levels -(Vdc1+Vdc2), -Vdc1,
1
ω
θ2
π −θ3
θ1
θ3
[ ∫ i(θ ) S2 (θ )dθ + ∫
+∫
π −θ1
π −θ2
-Vdc2, 0, Vdc2, Vdc1, Vdc1+Vdc2. No power transferring occurs
between H1 bridge and H2 bridge.
i (θ ) S2 (θ ) dθ
(3)
i (θ ) S2 (θ ) dθ ]
This unified equation can be applied to several special
(2) Situation 2: Energy Source 1 has enough available power
cases to solve the corresponding modulation index range.
to maintain the dc bus voltages Vdc1 near constant; the
•
available power of Energy Source 2 cannot maintain the dc
Pure resistive load
For a pure resistive load, the output current is proportional
bus voltages Vdc2 as required. The converter output 9 voltage
to the output voltage. In the interval θ3 ≤ θ ≤ π-θ3, the
levels –(Vdc1+Vdc2), -(Vdc1-Vdc2), -Vdc1, -Vdc2, 0, Vdc2, Vdc1,
capacitor is discharged by the load current, so the switching
Vdc1-Vdc2, Vdc1+Vdc2. Power is transferred from H1 bridge to
function is chosen to be –1 in the intervals θ1 ≤ θ ≤ θ2 and π-
H2 bridge to maintain H2 bridge dc bus to be Vdc2. Energy
θ2 ≤ θ ≤ π-θ1 to compensate the loss. In this case, (3) can be
Source 2 charges capacitor C2 during -(Vdc1-Vdc2) and (Vdc1-
formulated as
Vdc2) period to maintain a near constant dc bus voltage Vdc2.
Q=
Generally, in such a situation, the charging time decides the
ratio of the output power between energy source 1 and
energy source 2. In addition, during the duration when
1
1
3
[− vdc (θ 2 − θ1 ) + vdc (π − 2θ 3 )
2
ωR 2
1
− vdc (θ 2 − θ1 )]
2
(4)
voltage levels -(Vdc1-Vdc2) and (Vdc1-Vdc2) are produced,
where R is the load resistance. From the previous discussion,
Energy Source 1 and Energy Source 2 can charge capacitor
the capacitor voltage is maintained only if Q ≤ 0. The
C2 simultaneously.
switching angle must satisfy the following inequality
The modulation index in this cascaded system is defined
derived from (4):
as:
m=
3π − 6θ3 − 2θ 2 + 2θ1 ≤ 0
vfπ
(1)
2v dc
where vf is the fundamental magnitude of the output voltage.
The modulation index range for a pure resistive load is
therefore: 0 ≤ m ≤ 1.4
The modulation index m with two separate dc sources can
operate in the region of 0 ≤ m ≤ 2.52 [7]. The cascaded
•
Inductive load
inverter with a single dc source cannot achieve the same
Assume the high frequency harmonics are filtered by the
range. The higher modulation index means a higher output
inductive load, the load current can be regarded as a
voltage, resulting in the increased load current. The electric
sinusoidal waveform with the fundamental frequency, and it
lags the voltage by phase angle α , shown in Fig. 5.
charge on the capacitor drawn by the load cannot be
compensated by the dc source charging process, so the
normal operation cannot be maintained because of the
In order to find the modulation index range, the variation
of the electric charge on the capacitor must be studied first.
α
Vdc
Magnitude
capacitor voltage drop.
0
θ1 θ2 θ3
π
i
In the positive half cycle, it can be expressed as
Q=
1
ω∫
π
0
i (θ )S2 (θ )dθ ,
where i is the output current, and
(2)
ω
is the radian
0
0.002 0.004 0.006
0.01
time (s)
0.012
0.014 0.016
Fig. 5. Induction machine load current
frequency. S2(θ) here is the switching function for the Hbridge converter with capacitor. The value Q > 0 means the
0.008
Equation (3) becomes (5):
Wind Turbine
0.4
C p( λ )
PWM Rectifier
SCIG
Vw
ia*
ib*
Power coefficient
C
Gear
Box
ic*
Wr
Maximum
Power
Tracking
Control
Wr*
Iqs*
Rotor speed
Current control and
dq to abc
transformation
Controller
cos
Ids*
0.35
CP_max
0.3
0.25
0.2
0.15
0.1
0.05
λopt
0
0
2
sin
(a)
4
6
Tip speed ratio λ
8
10
(b)
Fig. 6. (a) The control block diagram of wind turbine-generator system, (b) power coefficient vs. tip speed ratio.
power flow
high-voltage side
low-voltage side
Cr1 C1
S1
Cr3
S3
v1
C3
+
v3
Tr
L1
V bat
v r12
i r12
iL1
Cr2 C2
S2
Iff*
(Pwind - P load)/V bat
V ref
+
Gv(s)
Ifb*
Cr4
S4
v2
S 1,2
P wind
Co
vr34
DC Bus
in1
C4
v4
-
S 3,4
PWM Drive
+
+
-
Voltage regulator
Gi(s)
Phi
Current Regulator
Fig. 7. Energy storage interface with dc bus using isolated bidirectional dual-half-bridge dc-dc converter.
1
Q=
ω
+
+
[
θ2
∫θ
cosθ 2 − cosθ1 + cos θ3 ≤ 0
I sin(θ − α )S2 (θ )dθ
The modulation index range satisfying (7) is 0 ≤ m ≤ 1.54 .
1
π −θ3
∫θ
I sin(θ − α )S2 (θ )dθ
3
(5)
π -θ1
B. Control of wind power and energy storage element
The operation principles of varied-speed wind turbine
∫π θ I sin(θ − α )S (θ )dθ ],
−
(7)
2
with battery are described in Fig. 6 and Fig. 7. Fig. 6 (a) is a
2
control block diagram of variable-speed wind turbine system
where I is the peak value of the current. The switching
where the rotor speed reference of the generator is designed
function here is again set to charge the capacitor, and (5) is
to extract the maximum power from the wind turbine at a
calculated as:
certain wind speed:
Q=
I
ω
[cos(θ 2 − α ) − cos(θ1 − α ) + cos(θ 3 + α )
+ cos(θ 3 − α ) + cos(θ 2 + α ) − cos(θ1 + α )]
=
where
2 I cos α
2 I cos α
ω
ω
[cos θ 2 − cos θ1 + cos θ3 ],
> 0 , since 0 < α < π / 2 .
Now the triangle inequality can be found as
ωr* = λopt
(6)
vw
rm
(8)
where Vw is wind speed, rm is turbine radius, and λ is the tip
speed ratio, λopt is the optimal λ of the maximum power
coefficient. A typical power coefficient curve versus tip
speed ratio of wind turbine is shown in Fig. 6(b).
Fig. 7 shows the control block diagram of battery
double loop control system with power feedforward
interfaced with dc bus using an isolated bidirectional dual
compensation is designed to achieve superior dynamic
half bridge dc-dc converter. The high frequency transformer
performance. The outer voltage loop detects dc link voltage
provides the electric isolation and voltage boost on the high
change and generates current reference Ifb* through a
voltage side (dc link). The converter works in buck mode to
voltage regulator. The feedforward current reference is
charge the battery and boost mode to discharge the battery.
calculated by:
Under normal operation, the dc-dc converter is controlled to
regulate the dc voltage to facilitate the flow of energy. A
I ff * = ( Pwind − Pload ) / Vbat
(9)
Wind maximum
power (W)
Wind speed (m/s)
(a)
(b)
DC link voltage (volt)
DHB inductor
Current (Amp)
(d)
(c)
Fig. 8. The simulated performance of wind turbine-battery system (a) changing wind speed, (b) the extracted maximum wind power, (c) the
input current of dual-half-bridge (DHB) dc-dc converter, and (d) the dc link voltage of H1 with battery voltage as 12 V, the desired dc link
voltage is 300 V.
(a)
(c)
(b)
(d)
Fig. 9. The simulated operation mode of hybrid energy system (a) capacitor voltage with resistive load, (b) output voltage with resistive
load, (c) capacitor voltage with inductive load, and (d) output voltage and current with inductive load when Vdc1=200V, Vdc2=100V.
80
V
60
V
a
Vc
b
Phase voltage (Volt)
40
20
0
-20
-40
-60
-80
0
0.005
0.01
0.015
Time (s)
0.02
0.025
0.03
(a)
(b)
Fig. 10. The experimental results of mixed operation mode of Energy Source 1 and 2 when Vdc1=48V and Vdc2=24V (a) three phase output
voltage, and (b) single phase output current
Pwind is the wind power in accordance with wind speed.
advantage of this power configuration and the corresponding
Pload is nominal power of the load. Vbat is the detected
control scheme is to improve the system survivability and
battery voltage, and in this case, the battery voltage variation
power quality. The simulation and experimental results are
could be also compensated by feed forward control. In
provided to verify the analysis.
addition, soft switching is achievable for this converter to
increase the efficiency.
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III. PERFORMANCE VERIFICATION
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IV. CONCLUSION
This paper studies a cascaded hybrid energy system that
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